The prefix bi generally means 2, as in biped or bipolar, so one means 2 times a year and the other means every 2 years, so not much help there.

Taking a look at the suffix, annual means yearly, and -ennial is, well, quite similar. A perennial plant is one which lasts a while, rather than something that shows up every year, so again we’re slightly in the dark.

As it turns out, biannual means twice a year, and biennial means every 2 years. I suppose you could say biennial is like triennial, which is once every 3 years, as long as you don’t think it means 3 times a year…hmmm.

I haven’t found a good way to remember which is which, other than the raw facts themselves, which is harder to do the older you get. Us older folk tend to learn via patterns rather than by rote these days.